An increased understanding of the cellular and molecular mechanisms that lead to cancer, has lead to an expectation that biological agents will play an increasing role in cancer therapy. The ability of a replicating virus to multiply its genetic "pay-load" up to several thousand-fold within a target cell, and then spread from cell-to-cell within a tumor, is an important characteristic to be harnessed in the development of a molecular therapy. However, in the few clinical trials to date replicating adenoviruses have only achieved limited clinical success. It has also become apparent that stroma plays a very important role in the development of tumors, and particularly so in pancreatic cancer which often has an intense stromal response. The foundation for this proposal is our recently published work showing that replicating adenovirus can persist for several weeks at high level within xenograft tumors without totally eliminating these tumors. This proposal is therefore directed towards understanding how the replicating adenovirus interacts with tumor cells, stromal cells and the tumor physiologic environment within pancreatic xenograft tumors. A better understanding of these factors may enable improvements to be made in the design of replicating adenoviral gene therapy vectors to facilitate their spread and efficacy in pancreatic tumors. Aim one will determine if stromal cellular or matrix components limit the spread of replicating adenoviral vectors through pancreatic xenograft tumors. Aim two will determine if areas of hypoxia within the tumor limit adenoviral replication. Aim three will determine if the intensity or permeability of the tumor vasculature influences viral distribution and spread within pancreatic xenograft tumors, and whether this can be modified for therapeutic advantage. Aim four will determine if xenograft tumor cells or the infecting virus change during the course of a persistent infection, and if these changes reduce the efficacy of the replicating adenovirus vector.